Paraffins, more commonly referred to as alkanes, are the chemical family of saturated hydrocarbons that result from combining CH2 groups in succession. Additional CH2 groups are added to form straight-chain paraffins.
The term “wax” simply refers to saturated hydrocarbons that contain more than 16 carbon atoms in the paraffin series (C16-C40) and are in a solid state at room temperature. The majority of waxes present in crude oil are considered synthetic paraffin waxes with non-oxidized saturated alkanes.
Paraffins may exist in crude oil in all three states. At standard room temperature, C16+n-paraffins generally exists in a solid form and solidify to form deposits. Wax is the product of paraffin deposition, so in the industrial context, “wax” and “paraffin” are often used interchangeably.
Therefore, “paraffin wax deposition” refers to the solid form of paraffins that solidify to cause deposition.
Because asphaltene is typically talked about in the same context as paraffin, it is important to understand what asphaltene is and why it is problematic to the crude oil service operations. Asphaltene is the material present in petroleum that is insoluble in n-paraffins but soluble in aromatic solvents. Asphaltenes cause catalyst deactivation and sediment formation.
Tars or asphaltenes occur in many crudes as colloidally suspended solid particles. Precipitation takes place when the crude loses its ability to keep those particles dispersed. Many of the same factors affecting paraffin deposition (discussed below) also affect asphaltene deposition.
Paraffins precipitate out of waxy crudes when there is a slight change in equilibrium conditions, causing a loss of solubility of the wax in the crude. A lowering in temperature is the most common cause of paraffin precipitation.
Thus, hydrocarbon liquids, including both crude oils and condensates, form a paraffin or asphaltene solid phase when process temperatures fall below the cloud point (or Wax Appearance Temperature) of the liquid. While this normally occurs in colder temperature services, it may also occur in any process where the combination of complex composition factors, such as API gravity, pressure/temperature variables and other factors favor deposition.
The presence of asphaltenes increases the difficulties for paraffin wax treatments because these structures are almost always found in association with waxes when they are retrieved from wells, storage tanks, or pipelines (Becker J. R. 1997, Crude oil waxes, emulsions, and asphaltenes. Tulsa, Okla.; PennWell Publishing Company). While paraffin wax deposition may be reduced by increasing the flow velocity of crude, increasing fluid velocities increases the likelihood of asphaltene deposition.
Further, studies show that the amount of asphaltene precipitation decreases as the number of carbons forming straight-chain paraffins increases. In other words, treatment of paraffin wax could escalate the precipitation of heavier, problematic asphaltene compounds. Therefore, treatment of paraffin and asphaltene deposition must both be considered in the balance.
Once formed, these paraffin/asphaltene solids will typically deposit on tank-mounted level sensors and instrumentation. This deposition presents a potential safety hazard when critical operational and/or safety sensors are affected. This may cause a loss of billions of dollars per year worldwide through the enormous cost of remediation, reduced or deferred production, well shut-ins, equipment replacements and/or abandonments, equipment failures, extra horsepower requirements, and increased manpower needs.
The modern petroleum industry has developed new technologies for controlling the deposition of petroleum paraffin and asphaltenes, particularly in wells, storage tanks, and pipelines. However, these technologies have been less effective on sensitive tank level sensors, flow sensors, and other instrumentation. Traditional methods of management and remediation have been established for many years and include the following:
a. Chemical Treatments and Additives: While chemical treatments help to manage solids deposition in connected lines, instrument tubing and storage tank internal components, some chemicals do not suspend the paraffin indefinitely and may be damaging to the environment.
b. Hot Oiling: Hot oiling is one method often employed for removing deposition in storage tanks. Paraffin and asphaltene buildup is handled by periodically pumping very hot oil, augmented by cleansing additives, into the vessels in order to melt the accumulations from tank walls, sensors and internal equipment.
c. Manual Cleaning: During normal maintenance operations, internally mounted equipment may be periodically subject to manual cleaning. This typically involves removal of sensors, instruments, etc. from out-of-service tanks and process equipment for cleaning. Alternately, personnel may physically enter out-of-service tanks to perform the cleaning with sensors and equipment in place. These methods typically involve considerable expense in time and labor for taking equipment out of service, the cleaning process itself, and management of safety associated with hazardous conditions, including hydrogen sulfide (H2S) exposure.
However, these procedures are labor and cost intensive and are not very effective with sensitive sensors and instrumentations. The solution is to avoid wax and asphaltene depositions in the first place.